in Thoreau’s Journal:

Passing up the Assabet, by the Hemlocks, where there has been a slide and some rocks have slid down into the river, I think I see how rocks come to be found in the midst of rivers. Rivers are continually changing their channels,  —eating into one bank and adding their sediment to the other, —so that frequently where there is a great bend you see a high and steep bank or hill on one side, which the river washes, and a broad meadow on the other. As the river eats into the hill, especially in freshets, it undermines the rocks, large and small, and they slide down, alone or with the sand and soil, to the water’s edge.

The river continues to eat into the hill, carrying away all the higher parts [of] the sand and soil, to add to its meadows or islands somewhere, but leaves the rocks where they rested, and thus in course of time they occupy the middle of the stream and, later still, the middle of the meadow, perchance, though it may be buried under the mud. But this does not explain how so many rocks lying in streams have been split in the direction of the current. Again, rivers appear to have travelled back and worn into the meadows of their creating, and then they become more meandering than ever. Thus in the course of ages the rivers wriggle in their beds, till it feels comfortable under them. Time is cheap and rather insignificant.  It matters not whether it is a river which changes from side to side in a geological period or an eel that wriggles past in an instant.